Install Tarsnap with apt-get

The installed version of tarsnap is 1.0.35, but the latest released version is 1.0.37.

Tarsnap has a [dpkg repository](https://www.tarsnap.com/pkg-deb.html). If we use apt-get to install tarsnap, I think we may be able to get automatic updates every evening with the rest of our system packages. Is there a reason not to do this?

Start by locating how this project currently installs or pins tarsnap and compare that setup with the linked dpkg repository and apt-get behavior described in the issue. Done means the installation uses the repository if appropriate and can receive updates through the system package workflow, with the project’s existing setup checks still passing.

Written by the indexing model from the issue text.